1. Field of the Invention
The present invention relates to an information recording medium in which recording, playback, and rewrite can be performed at high speed and high density using a phase change of a recording layer caused by laser irradiation.
2. Description of the Related Art
One of rewritable information recording media is a phase-change optical recording medium. A phase-change optical recording medium includes a recording layer in which the atomic arrangement reversibly changes between two different states (amorphous and crystalline states) by applying a light beam. Information is recorded by utilizing the two different states of the atomic arrangement of the recording layer. Phase-change optical recording media are particularly low priced among rewritable media, and therefore the consumer use thereof is significantly increasing. In particular, the use as home video recording media is rapidly increasing. By replacing video recording media, which have been tapes heretofore, with disks of phase-change optical recording media, new functions such as time-shift playback can be realized. Accordingly, more advanced characteristics are required than those required for phase-change optical recording media heretofore used as backup media for computers. For example, in the case of time-shift playback, since a just-recorded image needs to be followed and played back during recording, it is necessary to change between recording and playback at regular time intervals at high speed. In order to achieve this, it is necessary to further increase access speed for the recording and playback of information.
In a conventional phase-change optical recording medium, information is recorded and played back by controlling the number of revolutions of the medium by the constant linear velocity (CLV) technology. Since the CLV technology is a controlling method in which the relative velocity (linear velocity) between a light beam and the medium is always constant, the data transfer rate in recording and playback is always constant. Accordingly, it is possible to enormously simplify a signal processing circuit used for the recording and playback of information.
However, in the CLV technology, when the light beam moves in the radial direction on the medium, the number of revolutions of a motor needs to be adjusted according to the radial position of the light beam on the medium so that the linear velocity is maintained constant. Accordingly, the access speed for the recording and playback of information becomes low in the CLV technology.
On the other hand, in the constant angular velocity (CAV) technology in which information can be recorded and played back with the number of revolutions of a medium maintained constant, the number of revolutions of a motor does not need to be controlled according to the radial position. Accordingly, high-speed access can be realized.
However, in the CAV technology, since the data transfer rate in recording and playback varies depending on the radial position, a signal processing circuit used for the recording and playback of information becomes complicated. Further, in the CAV technology, since the linear velocity increases toward a peripheral portion of the disk, the crystallization speed of the recording layer in inner tracks have to be higher than that in outer tracks. Accordingly, in the CAV technology, a special recording layer is needed which has a crystallization speed ready for both of a high linear-velocity region in an outer portion of the disk and a low linear-velocity region in an inner portion thereof. In a phase-change optical recording medium, a Ge—Sb—Te alloy is generally used as a phase-change material for a recording layer. Specific examples include Ge2Sb2Te5, Ge6Sb2Te9, and Ge8Sb2Te11. The melting points of these are approximately 650° C. In order to reduce thermal damage to a substrate caused by heating such a phase-change material to a temperature equal to or higher than the melting point using a laser beam and cooling it in recording, protective layers made of dielectric material are often formed on both surfaces of the recording layer. Furthermore, a technology has been proposed which relates to an information recording medium having a structure that improves repeated-rewrite resistance by providing boundary layers between the recording layer and the protective layers, providing Cr2O3, Ge—N, GeCrN, or the like in the boundary layers, and thus preventing a chemical reaction between the recording layer and the protective layers and atom diffusion.
For example, according to Japanese Unexamined Patent Publication No. 2003-178487, a decrease in reflectivity caused by multiple rewrites can be suppressed by depositing a mixture of Ta2O5 and Cr2O3 with a thickness of 26 nm on an energy beam incident side of a recording layer.
(Patent Document 1) Japanese Unexamined Patent Publication No. 2003-178487
In known technologies, as phase-change recording layer materials with which high-speed recording can be performed, Ge—Sb—Te materials on the line connecting GeTe with Sb2Te3 in a triangle with vertices Ge, Sb, and Te are often used because of the high crystallization speeds thereof. Further, it has been known that the crystallization speed is further increased by substituting SnTe for GeTe in the above-described Ge—Sb—Te phase-change materials. GeTe has a melting point of 725° C., whereas SnTe has a high melting point of approximately 800° C. Accordingly, Ge—Sb—Sn—Te materials obtained by substituting SnTe for GeTe have higher melting points. In addition, a rewritable DVD medium in which the above-described high-speed recording can be performed is required to have performance in which recording is favorably performed even in conventional low-speed recording, in addition to high-speed recording. For example, in the case of a 4.7 GB DVD-RAM, the recording linear velocity (2× speed) defined in the specification for low-speed recording is 8.2 m/s. However, in a DVD-RAM ready for 5× speed recording, it is required that favorable recording performance can be obtained in a very wide linear velocity range of 8.2 m/s to 20.5 m/s. In order to satisfy this condition, the inventors of the present invention further studied Ge—Sn—Sb—Te materials obtained by substituting SnTe for GeTe in the above-described Ge—Sb—Te phase-change materials, by increasing SnTe to improve the crystallization speed. However, it was revealed that the above-described characteristics cannot be satisfied. Specifically, by substituting SnTe for GeTe, the difference in refractive indexes of the recording layer between crystalline and noncrystalline (amorphous) states is decreased with increasing Sn. Moreover, the crystallization speed of the recording layer becomes too high at low linear velocities. For this reason, in a cooling process immediately after heating the recording layer to a temperature equal to or higher than the melting point using a laser beam, crystals grow from an outer edge portion of a melted region, and recrystallization therefore occurs, which reduces a recording mark size. Thus, there arises the problem that a reproduced signal is reduced.
In order to solve these, the inventors of the present invention further studied phase-change recording layer materials of BiGeTe which are suitable for speed enhancement. According to an experimental study by the inventors, for example, the composition of Bi7Ge43Te50 suitable for speed enhancement is a composition in which Ge is excessively added more than those on the line connecting GeTe with Bi2Te3 in a composition triangle with vertices Bi, Ge, and Te. It has been proved that such compositions can satisfy the aforementioned characteristics in a wide range from low-speed recording to a high-speed recording. The reasons for this are the following: activation energy for crystallization is large in the vicinity of Ge50Te50 in the aforementioned composition triangle, and the stability of an amorphous mark increases in low-speed recording; and high-speed crystallization is allowed because an appropriate amount of BiTe is added. However, the melting point of such a composition is at least 700° C. or more, which is higher than approximately 650° C. of conventional Ge—Sb—Sn—Te phase-change materials by approximately 50° C. Accordingly, in a phase-change optical recording medium in which a phase-change material in this composition region is used, excellent rewrite resistance, shelf life, and reproduced-signal output characteristics will probably be very hard to obtain if Cr2O3 or GeCr—N is used, which have been heretofore used in low melting-point phase-change recording media.
Against this background, the inventors of the present invention prepared information recording media in which the aforementioned boundary layers and phase-change recording film materials with high melting-point were used, and investigated multiple-rewrite performance, shelf life under a humidified environment, and reproduced-signal output characteristics. As for reproduced-signal output characteristics, favorable characteristics could be obtained by optimizing the amount of added Bi among compositions in which Ge is excessively added more than those on the line connecting GeTe with Bi2Te3 in a composition triangle with vertices Bi, Ge, and Te. However, it was proved that multiple-rewrite performance is favorable in materials except Cr2O3. In Japanese Unexamined Patent Publication No. Hei 10(1998)-154352, it is described that multiple-rewrite performance is improved by using Cr2O3 for a boundary layer. However, an experiment by the inventors revealed that multiple-rewrite performance deteriorates in the case where a high melting-point material is used as a recording film material. On the other hand, as for shelf life in a humidified environment, it was proved that only Cr2O3, which is inferior in multiple-rewrite performance to others, shows favorable characteristics.